Massive multiple-input and multiple-output (MIMO) and In Band Full Duplex (IBFD) wireless communication are two very prominent techniques being studied under the fifth generation (5G) of wireless technologies. For Massive MIMO systems with large number of antennas at a transmitter, optimal gain is achieved through beamforming by the transmitter towards an appropriate receiver. For instance, the transmitter is a Massive MIMO base station and receivers could be multiple antennas or multiple user equipment spread in a given geographical area. For efficient beam forming, the receiver needs to feedback channel information for large number of antenna links, thus consuming more power and bandwidth.
Typically, the IBFD system transmits and receives data simultaneously using the same channel. In case of the IBFD systems, the power difference between a received signal and self-interference generated by the transmitted signal (up to 130 dB for cellular systems) makes the process of data detection difficult. For Full Duplex (FD) systems, the huge power difference between the received signal and the self-interference generated by the transmitted signal (up to 130 dB for cellular systems), makes the task of data detection difficult. The situation becomes even worse for higher order modulations.
Generally, heterogeneous networks are one of the most preferred ways of increasing network capacities to meet the demands of future networks. A network composed of various layers of cells results in increased spectral efficiency and reduced coverage holes. Cell Range Expansion (CRE) or biasing a user to associate with a smaller tier to offload traffic from macro cells is a well-established technique. Though the capacity of the network as a whole increases, lot of the CRE users experience a bad Downlink (DL) signal-to-interference-plus-noise ratio due to heavy interference from a Macro Base Station (M-BS). For such CRE users, the DL channel from the M-BS is better from their associated small base station (S-BS). For such users, maintaining dual connectivity is optimal i.e., the DL from the M-BS and Uplink (UL) to the S-BS.
Conventionally such CRE users or devices need to duplex between these two connections either in time, frequency, code, and space. With IBFD User Equipment (UE), the need for duplexing between the two connections is obviated. Thus, by using the IBFD UE, the dual connections are maintained on the same channel, simultaneously, thereby resulting in overall increase in the spectral efficiency of the network. There is a tradeoff between form factor and self-interference cancellation (SIC) capability of the device. Further, the IBFD UE with a small form factor has much less SIC capability than the IBFD BS.
High bandwidth demands expected from 5G (5th generation wireless systems) and future wireless networks impose tight constraints on channel resource usage. The resources being available in the form of frequency bands. This has resulted in wireless stations operating on channels becoming increasingly close to each other in frequency. For instance, this separation in state-of-the-art 3GPP Long term evolution (LTE) dynamic time-division duplex (TDD) networks could be as low as 2.5 MHz. This means that a station could be receiving information on a carrier frequency that is only 2.5 MHz apart from the allocated transmit frequency of a neighboring station. The radio-frequency (RF) filters used at the transmitting station cannot practically have brick wall spectrum shaping resulting in spectral leakage out of the allocated spectrum of the transmitter to the adjacent receiver's spectrum.
Hence, due to the limited out-of-band suppression provided by the RF filters, a transmitting station is a potential interference source for neighboring receiver stations operating on adjacent frequencies. This out-of-band interference that is generated by a transmitting station to a close-by (in frequency) receiving station is termed as adjacent channel interference (ACI). Mitigating the adverse effect of the ACI is important for a communication system, as it could potentially saturate the receiver, preventing it from decoding the received signal.